Pass base uri to serializer when writing to file.

[sandervd]

Discussion in #2976

Summary of changes

Also pass base uri to to the serializer when the destination is set.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• If the change adds new features or changes the RDFLib public API:
  • Created an issue to discuss the change and get in-principle agreement.
  • Considered adding an example in ./examples.
  • Added or updated tests that fail without the change.
  • Updated relevant documentation to avoid inaccuracies.
  • [ ]~~ Considered adding additional documentation.~~
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.